Metal-air battery including fibrillated cathode



19, 1969 J. D. VOORHIES AL 3,462,307

METAL-AIR BATTERY INCLUDING FIBRILLATED CAT HODE Filed April 28, 1966 2Sheets-Sheet 1 INVENTORS. JOH/V DA V/DSO/V VOORH/ES HEA/{f Y PA TRICKLAND/ Aug. 19, 1969 Q J VQQRHlES ET AL 3,462,307

METAL-AIR BATTERY INCLUDING FIBRILLATED CATHODE Filed April 28, 1966 2Sheets-Sheet 2 INVENTORS, JOHN DAVIDSON vaonmss HENRY PA TRICK LAND/ A7' TORNEY 3,462,307 METAL-AER BATTERY TNCLUDKNG FHBRILLATED CATHODE JohnDavidson Voorhies, New Canaan, Conn, and Henry Patrick Landi, YorktownHeights, N.Y., assignors to American Cyanarnitl ornpany, Stamford, Conn,a

corporation of Maine Filed Apr. 28, 1966, Ser. No. 546,t)1l2 Int. Cl.Htllrn 27/06 US. Cl. l3686 l tClairns The present invention relates toan improved alkaline metal-air battery. More particularly, it relates toan improved alkaline metal-air battery utilizing a novelgraphite-filled, extensively fibrillated, porous polytetrafiuoroethyleneelectrode structure as a cathode and has as its prinicpal object theutilization of such battery as an inexpensive power source.

Alkaline metal-air batteries have been long known. However, suchbatteries have not been extensively used in the art, since they aregenerally bulky, extremely heavy and possess relatively low powerdensities. Where excessive weight, large bulk and low energy densitiescan be tolerated, such batteries are found to be acceptable. In recentyears, there has developed a real need for a relatively light weight,high energy density battery because of the advent of battery poweredtoothbrushes, electric shavers, tools and even vehicles. The provisionof an alkaline metal-air battery would fulfill this need, if both itsoverall weight could be decreased and its power density be substantiallyincreased.

It is, therefore, a principal object of the invention to provide animproved alkaline metal-air battery of enhanced power density andrelatively low weight. It is a further object to provide a metal-airbattery of enhanced power density and of extremely low overall weightand volume when utilizing a novel, highly porous, graphite-filledpolytetrafiuoroethylene electrode. Other objects and advantages willbecome apparent from a consideration of the following description.

To this end, there is provided a metal-air battery of relatively lowweight, volume and enhanced power density. This is unexpectedlyaccomplished by incorporating a highly porous, graphite-filled,extensively fibrillated electrode which permits the ready transfer ofair through said electrode while retaining liquid electrolyte within aconfined area. The use of the novel electrode structure in conjunctionwith an electrode comprising a suitable metal, such as zinc, enhancesthe overall power density of a battery prepared therefrom, whilesimultaneously reducing substantially the overall total weight andvolume.

According to the invention, there is provided either a single cell or,preferably, a battery of cells, each cell comprising essentially acathode of a graphite-filled polytetrafluoroethylene, hereinbelowdescribed with particularity. This cathode abuts an area constituting areservoir in which there is confined an aqueous electrolyte. There ispositioned an electropositive metal which directly contacts theelectrolyte retained in the area provided. The opposite face of thecathode is provided with a metallic air spacer. The latter consists, inone embodiment, of a metallic structure of sufiicient thickness topermit air to permeate therethrough and, finally, through the cathode.On the anode side of the cell, there is positioned a metallic spacerwhich abuts a metallic sheet. If desired, there is further provided ametallic screen which faces the metal anode so that contact of theelectrode is made with the metal sheet. Electric current is passed fromthe anode to the steel sheet. Thus, there is provided a bipolarelectrical contact in a multicell system.

On the addition of alkaline electrolyte to the above confined area,current flow can be detected by means of an ammeter by attaching aterminal wire lead from the metallic spacer on the cathode side to aterminal wire lead attached to the metallic sheet on the anode side ofthe cell.

In another embodiment, the aforementioned metallic air-spacer need notcomprise a plurality of metallic screens. Rather, the spacer can befabricated as a metallic ring structure having a plurality ofprotrusions on one face thereof and its opposite face is provided with asmooth surface which directly abuts the cathode as hereinabove defined.

In general, the anode employed in the cell of the present invention isan electropositive metal. Exemplary of the latter are: zinc, barium,magnesium or aluminum. There is also contemplated mixtures or alloys ofthe aforementioned metals including such other metals as manganese,calcium, cerium, mercury and zirconium. However, for optimum operation,zinc in its pure or alloyed form is the preferred metal anode.

The cathode of the present invention comprises a highly porous,extensively fibrillated, graphite-filled polytetrafiuoroethylenestructure. The latter is gas permeable but water impermeable. Inpreparing such structure, polymethylmethacrylate is heated and milled toa molten, viscous state at a temperature between about C. and about C.Polytetrafiuoroethylene, in the form of an aqueous dispersion of finelydivided particles, is gradually added to the moltenpolymethylmethacrylate, thereby forming interconnected and interwovenfibrils of polytetrafiuoroethylene. As milling is continued, water isvolatilized from the dispersion and graphite or any equivalent thereofis next incorporated into the milled mixture. The blend is removed fromthe mill and ground into pellets which are directly extruded byconventional techniques. The latter are subsequently compression moldedand a sheet of any desired size, shape or form is thereby obtained.Thereafter, the polymethylmethacrylate is extracted from the resultantsheet structure by means of several washings with acetone. The lattersolvent selectively extracts from the sheet all thepolymethylmethacrylate present. Further, separate washings of theso-extracted sheet with ethyl alcohol and Water, respectively, areperformed and then the sheet is rolled between, for instance, blotterpaper so as to dry it. If desired, an active catalyst, exemplary ofwhich are platinum, palladium, copper, silver and a silver-mercuryamalgam, can be incorporated directly into the so-treatedpolytetrafluoroethylene sheet during the milling step. One other methodfor accomplishing the incorporation of an active catalyst is, forinstance, by impregnating the sheet in an alcoholic solution ofchloroplatinic acid, followed by thermal degradation of the latter acidin the presence of hydrogen. However, this platinization or metalactivation procedure is not a critical operation in the preparation ofthe cathode employed in the aforementioned metal-air battery of thepresent invention.

The added elctrolyte solution is alkaline. Preferably, from about a 6 Nto about 10 N aqueous potassium hydroxide solution is introduced intoeach cell. Other equivalent electrolytes, such as sodium hydroxide, orlithium hydroxide, can also be utilized, if desired.

In order to further clarify the invention, there is shown in theaccompanying drawings preferred embodiments of the metal-air battery andthese Will be descibed in detail.

In the drawing:

FIGURE 1 is an exploded plan view, partially in section, of an alkalinemetal-air battery employing the novel cathode hereinabove defined;

FIGURE 2 is a partially expanded side view, partially in section, of thebattery of the present invention; and

FIGURE 3 is an alternative embodiment of an exploded plan view,partially in section, of a cell which comprises a single unit of theoverall battery of the invention.

In FIGURE 1, there is shown a port I for the introduction of electrolyteinto a confined area 2. If desired, it may be closed by an appropriatesealer. This area is determined by a polyethylene electrolyte chamber 3.This ring is so designed as to receive in its recess a gasket 4 whichsnugly houses a metal anode electrode, such as zinc 5. Directlycontacting the electrode 5 is a metal screen 6 which is supported withina second sealing gasket 7. In contact with and facing the screen is ametallic sheet 8.

On the opposite face of the polyethylene electrolyte chamber 3 ispositioned a graphite-filled, highly porous, extensively fibrillatedpolytetrafiuoroethylene cathode structure 9. Abutting the opposite faceof the cathode 9 are spaced elements 10 adapted for the diffusion of airwhich comprise a metallic screen 11, a metallic ring 12 which positionsa metallic screen 13. The latter screens are of size and shapesuflicient to overlap the metallic ring 12. When all the elements arepressed together, resultant cell takes the shape of a thin wafer. Aplurality of the latter thin wafers comprises a battery.

In FIGURE 2, there is depicted a battery comprising three compressedcells. There is shown a vent 21, 21a and 21b for the insertion ofelectrolyte. Electrolyte chamber 22, 22a and 22b holds introducedelectrolyte solution, said chamber being confined by a polyethyleneelectrolyte chamber or ring 23, 23a and 23b. The electrolyte directlycontacts a suitable electropositive metal 24, 24a and 24b surrounded atits periphery by a ring gasket 25, 25a and 25b. The metal electrodeabuts a metallic screen 26, 26a and 2617, which screen is surrounded bya gasket 27, 27a and 27b. Finally, a metal sheet 28, 28a and 28b abutsthe metallic screen so as to confine on one face of the polyethyleneelectrolyte chamber any electrolyte solution fed into the chamber. Toconfine the electrolyte completely, the cathode structure 29, 29a and2911 located opposite the anode side is positioned to contact the faceof the polyethylene electrolyte chamber. Bipolar connecting air spacercomprises metal screen 31, 31a and 31b, as well as metal foil 32, 32aand 32b, to which a metal screen of either a single or double thicknessis provided at 33, 33a and 33b.

Resultant cell units are compressed and at one extremity terminal leadwire 34 is provided and at the opposite terminal end lead wire 35 isprovided. The three units are compressed by means of plastic plates 36and 36a secured by bolted studs 37 and 37a and nuts 38 and 38a.

Advantageously, internal connectors are entirely eliminated. Moreover,since the cells are bi-polar, current is caused to pass directly thruoghthe several cells with minimum resistive loss. Thus, the efficiency ofthe overall battery is markedly enhanced.

In FIGURE 3, another embodiment of the invention is shown. This issubstantially the cell corresponding to FIGURE 1. Advantageously, theelements of air spacer 10 in FIGURE 1 is replaced by a singleair-metallic spacer 50. Hence, in FIGURE 3, there is shown a port 41through which electrolyte is introduced into a confined chamber 42surrounded by a polyethylene electrolyte chamber 43 which is slightlyrecessed on one face to receive gasket 44. The latter houses a zinc orother suitable metal electrode 45. Abutting the metallic electrode 45 isa screen 46, which snugly fits into a gasket 47 and simultaneouslycontacts electrode 45. Facing the gasket directly on one of its faces isa metallic sheet 48 which contacts screen 46. On the opposite face ofthe polyethylene electrolyte chamber 43 is positioned a graphite-filled,highly porous, extensively fibrillated cathode or electrode 49.Electrolyte is thereby retained in the reservoir 42. The face oppositethe cathode 49 abuts a ring spacer 50 in which one surface thereof iscompletely smooth and the other face is provided with protrusions orprojections 50a 4 through 50h, inclusive. The protrusions thus allow forair to pass through the ring spacer to contact the porous electrode 4-9.

In a specific embodiment, there are employed three cells as in FIGURE 1above, each cell containing a reservoir filled with 6 N potassiumhydroxide. In order to prevent the solution from dripping out of theports into which the alkaline electrolyte solution is added, a suitablesealer,

such as a plastic cap, tar or rosin, is applied to the opening. Themetallic anode is pure zinc and the metal screens Which contact theanode consists essentially of nickel. The active component of the aircathode which measures one inch in diameter is uncatalyzed graphite. Avolume of 1.14 cubic inches and an overall battery weight of 0.146 poundcomprise the physical measurements of the compressed three cells. Whensupplying air by means of natural convection at the rate of twenty cubiccentimeters per minute, a maximum power of 1.0 watt and a maximum powerdensity equal to 0.71 watt per cubic inch or 6.85 watts per pound arenoted. At 2.0 volts and 2.3 volts, respectively, there is also obtaineda total corresponding current of 0.34- ampere and 0.21 ampere.

Another specific embodiment of the invention involves the modificationof the battery comprising three cells described above. Improvedperformance can be obtained by catalyzing the graphite, such as bydepositing thereon platinum, copper, silver, silver-mercury amalgam,usually in the range of 50%75% silver to 50%25% mercury, or palladium.The amount of catalyst deposited may widely vary, preferably frombetween 1.0 milligram and 20 milligrams of catalyst per squarecentimeter of electrode surface. Deposition of the catalyst on thegraphite can be carried out by various techniques, such as by in situreduction of chloroplatinic acid utilizing either diphenyl silane orsodium borohydride to precipitate elemental platinum on said graphite.In the modification of the air-electrode, there are prepared twocatalyzed graphite electrodes, one containing ten milligrams of silverper square centimeter and the other containing 2.5 milligrams per squarecentimeter of platinum. These electrodes are used in lieu of theuncatalyzed graphite electrode in an identical manner as described inthe specific embodiment above. The improved performance results arereported in the table below.

TABLE Catalyzed Catalyzed electrode electrode (10 mgJcm. (2.5 rug/cm.Characteristics of the Battery Ag) Pt) Number of cells Electrolyte 6NKOH 6N KOH Total volume (cu. in.). 1. 4 1. 4 Total Weight (11).) 0. 1460. 146 Method of air supply Air electrode, diameter (inch) 1 1 Maximumpower (watt) 1. 27 1. 32 Maximum power density:

Watts per cu. in 0. 9O 0. 94 Watts per pound 8. 7 9.1 Total current at2.0 volts (a. 0. 68 0.68 Total current at 2.3 volts (at). 0.55 0. 58

1 Natural convection.

Utilizing silver-mercury (%25% copper or palladium in lieu of platinumor silver as the electrode catalyst, substantially the same highperformance results are realized.

To demonstrate the marked improved performance of the battery of thepresent invention, the well-known prior art Edison Carbonaire batterywhich contains two cells and utilizes an alkaline electrolyte iscompared. In this regard, there is noted that the Edison batteryoccupies a total volume of 500 cubic inches and weighs 28 pounds. Whensupplying air to it by natural convection, there is obtained a maximumpower of 4 watts and a power density of only 0.008 watt per cubic inchor 0.143 watt per pound. Thus, the Edison Carbonaire battery is found topossess markedly increased total weight and volume as compared to thedescribed battery of the present invention. Further,

it appears to lack the corresponding increased power density possessedby the battery of the invention.

Advantageously, the battery of the present invention can be rechargedand, alternatively, electrolyte, if desired, can be Withdrawn therefromand fresh electrolyte reintroduced. Further, because of its small sizeand weight and high power density, the metal-air battery of theinvention can be used effectively and efiiciently in portable emergencylighting apparatus and in providing the power required to drive powerequipment, such as shavers, toothbrushes, portable power drills and evenautomobiles. Further, the battery of the present invention can be shapedor formed as desired. For most purposes, however, it is contemplatedthat the design of the battery be in a cylindrical form suitbale forincorporation into powered equipment.

We claim:

1. A metal-air battery of the bi-polar type having positioned thereinterminal electrical connectors comprising in combination: a plurality ofelectrically linked compressed cells, each of said compressed cellsconsisting essentially of: (a) a metallized air spacer, (b) agraphitefilled, highly porous, extensively fibrillatedpolytetrafluoroethylene cathode structure which is gas permeable butaqueous electrolyte impermeable, (c) an insulated container adapted tobe filled with an aqueous electrolyte, (d) a metal anode, saidgraphite-filled cathode and metal anode directly contacting electrolytein said container, and (e) means for electrically connecting said metalanode of one cell with said metallized air spacer of an abutting cell.

2. The battery of claim 1 in which the metallic anode is zinc.

3. The battery of claim 1 in which the metallic screen is nickel screen.

4. The battery of claim 1 in which the electrolyte is potassiumhydroxide.

5. The battery of claim 1 in which the electrolyte is sodium hydroxide.

6. The battery according to claim 1 in which the graphite-filled cathodeis further platinized.

7. The battery according to claim 1 in which the graphite-filled cathodeis further copperized.

8. The battery according to claim 1 in which the graphite-filled cathodeis further silverized.

9. The battery according to claim 1 in which the graphite-filled cathodeis further palladized.

10. The battery according to claim 1 in which the graphite-filledcathode is further treated with a silvermercury amalgam.

References Cited UNITED STATES PATENTS 3,043,898 7/1962 Miller et al136-86 3,328,205 6/1967 Barber et al. 136-86 3,348,974 10/1967 Barber eta1. 13686 ALLEN B. CURTIS, Primary Examiner UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,L 62,307 Dated August 19, 1969Inventor(5) JOHN D and HENRY P.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

F Column l, line 69, cancel "steel" and substitute metal Column 2, line58, insert alkaline before metal-air".

Column 3, line 19, cancel "spaced" and substitute spacer GU57 1; SEALEDAttest:

Edw WILLIAM E. SGHUYLER, JR Attesting Offi r Commissioner of Patents

1. A METAL-AIR BATTERY OF THE BI-POLAR TYPE HAVING POSITIONED THEREINTERMINAL ELECTRICAL CONNECTORS COMPRISING IN COMBINATION: A PLURALITY OFELECTRICALLY LINKED COMPRESSED CELLS, EACH OF SAID COMPRESSED CELLSCONSISTING ESSENTIALLY OF: (A) A METALLIZED AIR SPACER, (B) AGRAPHITEFILLED, HIGHLY POROUS, EXTENSIVELY FIBRILLATEDPOLYTETRAFLUOROETHYLENE CATHODE STRUCTURE WHICH IS GAS PERMEABLE BUTAQUEOUS ELECTROLYTE IMPERMEABLE, (C) AN INSULATED CONTAINER ADAPTED TOBE FILLED WITH AN AQUEOUS ELECTROLYTE,